//Given a binary tree, return the postorder traversal of its nodes' values. 
//
// Example: 
//
// 
//Input: [1,null,2,3]
//   1
//    \
//     2
//    /
//   3
//
//Output: [3,2,1]
// 
//
// Follow up: Recursive solution is trivial, could you do it iteratively? 
// Related Topics 栈 树


//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Example:
 * var ti = TreeNode(5)
 * var v = ti.`val`
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */
class Solution {
    fun postorderTraversal(root: TreeNode?): List<Int> {

    }
}
//leetcode submit region end(Prohibit modification and deletion)
